Update V8 to r6387 as required by WebKit r76408

Change-Id: Icfc5385b0996bd592f8b1ac8cbb44767ee09f1f6
diff --git a/src/x64/code-stubs-x64.h b/src/x64/code-stubs-x64.h
index 5056f34..9feced2 100644
--- a/src/x64/code-stubs-x64.h
+++ b/src/x64/code-stubs-x64.h
@@ -198,6 +198,7 @@
   }
 
   friend class CodeGenerator;
+  friend class LCodeGen;
 };